1. Field of the Invention
The present invention relates to a drive control system of a four-wheel drive vehicle in which all of the front and rear wheels are driven and, more particularly, to a slip control of the four-wheel drive vehicle.
2. Description of Related Art
A four-wheel drive vehicle has a center differential gear interposed between front and rear wheels. The disposition of the center differential gear enables the four wheels to drive the vehicle in a stable fashion because a "tight corner brake" phenomenon or the like can be avoided by its differential action.
In instances where the center differential gear is disposed in the four-wheel drive vehicle, however, the problem may arise that driving performance cannot be achieved to a sufficient extent because a total driving force is reduced due to the differential action of the center differential gear, when either of the driven wheels would slip excessively or either of them skids during running on a slippery road or the like.
In order to achieve a sufficient degree of driving performance of the four-wheel drive vehicle, a differential restricting device such as a clutch has been disposed together with the center differential gear, thereby operating the differential restricting device so as to couple the front wheels directly with the rear wheels when the vehicle slips to such an extent that the difference between a from wheel speed (a peripheral front-wheel speed) and a rear wheel speed (a peripheral rear-wheel speed) reaches a value which is equal to or greater than a predetermined value. On the other hand, in other running states, the operation of the differential restricting device is released to thereby allow an operation of the center differential gear.
Japanese Patent Unexamined Publication (kokai) No. 261,539/1987 discloses a four-wheel drive car with such a center differential and a differential restricting device for restricting the operation of the center differential. The technology involved in this four-wheel drive system is arranged such that a microcomputer is adopted for a control unit and data or information on the wheel speeds of the wheels, the steered angle of the steering wheel and so on is inputted into tee control unit as well as wheel speeds of the front and rear wheels and a target speed difference are determined from maps, thereby performing a slip control on the basis of a feedback control of the difference restricting device so as to allow a difference between actual speeds of the front and rear wheels to approach to the target speed difference. In other words, the technology disclosed in this prior patent publication involves the feedback control of the differential restricting device by using the difference of the wheel speeds of the front and rear wheels as a control target value which varies with the steered angle of the steering wheel.
U.S. Pat. No. 4,723,624 discloses the technology which involves turning the clutch as a differential restricting device off when the difference between the wheel speeds of the front and rear wheels reaches a predetermined value.
It is to be noted herein that an object to be controlled originally in a control of the four-wheel drive car is a slip ratio S of the wheels, which functions as one of the big factors governing the behavior of the vehicle body and it is well known that a friction coefficient of a wheel against the surface of a road or a cornering force CF may vary to a great extent with the slip ratio S. Hence, it is most preferred to set the slip ratio S itself as a control target value in controlling the four-wheel drive car. The slip ratio S may be defined herein as follows: ##EQU1##
The adoption of the slip ratio S as the control target value, however, presents the following problem. As microcomputers to be used for the control units for the four-wheel drive system for automotive vehicles are generally said to be rather slow in processing the division, the processing of the division may accordingly take a little longer period of time for giving an actual slip ratio S to be compared with a target slip ratio S.sub.T when the slip ratio S which is obtained by the division for giving the actual slip ratio S is set as a target value, thereby causing a delay in performing the control.
Therefore, it is common in conventional technology that the speed difference obtainable by the processing of the subtraction is adopted as a control target value in order to avoid the delay in performing the control. It is to be noted herein that the speed difference (the rotational speed difference between the front and rear wheels) as the control target value is adopted merely as a matter of convenience and it is different in concept from the slip ratio S that should originally be adopted as the control target value.
More specifically, on the one hand, suppose that a speed difference between the wheel speed of the front wheel and the wheel speed of the rear wheel is 10 km per hour while the wheel speed of the rear wheel is approximately equal to the vehicle speed as high as 100 km per hour. In this instance, the slip ratio S is determined to be 0.1. On the other hand, suppose that the speed difference between the wheel speeds of the front and rear wheels is the same as above while the vehicle speed is as high as 50 km per hour, then the slip ratio S is determined to be 0.2. In summary, the slip ratio S may vary even if the speed difference between the front and rear wheels would be the same. In other words, the slip ratio may have the different meaning according to a running state of the vehicle even if the speed difference between the front and rear wheels would be the same.
Japanese Patent Unexamined Publication (kokai) No. 11,431/1988 discloses another technology which uses a speed ratio of the wheel speed of the front wheel to the wheel speed of the rear wheel as a target value for the slip control. This slip control involves a feedback control by comparing the actual-speed ratio with the target speed ratio. It is to be noted herein that the speed ratio adopted in this conventional technology is approximately the same in concept as the slip ratio in performing the slip control. It is to be noted, however, that the speed ratio is to be given by the division of the wheel speed of the front wheel by the wheel speed of the rear wheel, so that the operation processing of the speed ratio requires a time period as long as the slip ratio S, thereby incurring the risk of causing the delay of control. Further, this technology is devised so as to give the target speed ratio from a map so that a large number of maps are prepared so as to correspond to various running conditions in order to provide control characteristics over a wide range containing a variety of running states of the automotive vehicle, such as an "ON" state and an "OFF" state of an anti-lock brake system (ABS system), a tight corner braking requiring a large steered angle of the steering wheel, occurrence of a stuck state of the vehicle, and so on.
Conventionally, the slip control is generally performed by implementing the setting of the target value and the following feedback control in a series of routines, so that a quick control has strongly been desired because a large number of processing steps are required in order to perform the slip control accurately so as to comply with various running states of the vehicle.